Ferroelectric imaging system

ABSTRACT

An improved ferroelectric imaging system comprises a chopper, lens system, ferroelectric detector matrix, switching matrix, drive and readout electronics, video processor and display. The chopper interrupts infrared energy emanating from a scene, and the lens system focuses the chopped infrared energy on the ferroelectric detector matrix which produces electrical signals representative of the impinging infrared energy. The signals are read out by the drive and read out electronics whose action is synchronized with the chopper action, processed in the video processor and displayed by the display. The ferroelectric matrix comprises a plurality of detector capacitor elements fabricated by forming a metal surface on the upper surface of a dielectric of ferroelectric material and a plurality of metal pads arranged in rows and columns on its lower surface. Said metal plate forming the upper plates for the capacitors and the metal pads forming the lower plates. The upper plate is connected to busbars for biasing. The switching matrix includes a plurality of solid state switches arranged in rows and columns. The switches have elements operatively connected to the lower plates of the detector capacitor elements, a sense line for each row, and x and y address lines of the drive and readout electronics. The drive and readout electronics action is synchronized with the chopper action to provide the sense line with a signal representative of the infrared energy impinging sequentially on each detector capacitor element for processing and display.

This invention relates to thermal imagers, and more particularly, to animproved ferroelectric imaging system.

In the past, ferroelectric imager systems have included a lens system, adectector matrix, drive and read out electronics, a video processor, atiming and control means and a display means. The detector matrixincludes a slab of ferroelectric material having orthogonal leadpatterns formed respectively on opposing major surfaces of theferroelectric material. The intersection of the orthogonal leadpatterns, which constitutes plates, together with the ferroelectricmaterial, which constitutes a dielectric, form capacitor type elementswhich make up the detector matrix. The capacitors are temperaturedependent and are the basis for the ferroelectric transducer. Soconstructed, the detector elements of the detector matrix are in rowsand columns. The detector matrix is positioned within the housing sothat all but one row are in front of the lens system to receive theinfrared energy passing therethrough. The remaining row is shielded frominfrared energy passing through the window to provide a row of referencedetector elements.

The detector matrix is coupled to the drive and read out electronics.The drive electronics include a scan multiplexer and a pair of sine wavesignal generators. The reference row of detector elements are connectedto a first one of the sine wave signal generators while the remainingrows of detector elements are connected to the second of the sine wavesignal generators through a switch and multiplexer. The read outelectronics comprises preamplifiers connected to the columns of detectorelements and whose outputs are connected to a signal multiplexer. Thesignal multiplexer is connected to the video processor which isconnected to the display.

In this arrangement the IR lens system focuses infrared image of a sceneonto the detector matrix. Then, the drive scan multiplexer, upon commandof the timing and control means, connects sequentially the second sinewave generator to the rows of detector elements. The generator outputcharges in turn each row of detector elements. The charge of eachdetector element of each row is varied by the intensity of thermalenergy impinging thereon. The reference detector elements connected tothe columns of detector elements are charged by the generator. Thesignal multiplexer of the read out electronics, upon command of thetiming and control mechanism, sequentially connects each element of arow of detector elements to the video processor.The output signals ofthe reference elements are combined with the outputs of the rows ofdetector elements and signals equal to the difference of the charges areproduced for amplification by preamplifiers decoded by a decoder andpassed through the video processor. The read out multiplexer similarlyconnects sequentially the detector elements of the detector to producethe video signals representative of the scene for display on the cathoderay tube. The above described ferroelectric imaging system is more fullydescribed in U.S. Patent Application Ser. No. 796,785, filed May 13,1977, now U.S. Pat. No. 4,080,532.

The prior art system suffers many disadvantages. Examples ofdisadvantages are: shunt capacitance which exists along any column ofdetector elements because of the capacitance of each detector elementalong that column and in a nonaddressed row; a fixed pattern noise whichexists because of element to element capacitance uniformity; and athermal short which can exist between the detector element and thesubstrate because support thermal impedance is not provided.

Still other disadvantages exist, when the upper capacitor surfaces areused as the sense lines, the sense lines must be ball bonded down offthe upper detector surface to the individual preamps or to individualswitches. The multiball bonds are undesirable because of yield andtechnique complexity. Also the sharing of a single preamp to reduce thenumber of preamplifiers is objectional because of the magnitude of theparasitic capacitance. Further, the conductive strip forming the senseline has the electrical characteristics of a transmission line; thesecharacteristics restrict the speed with which any one line or detectorcan be addressed.

Accordingly, it is an object of the present invention to provide animproved ferroelectric imaging system.

Another object of the invention is to provide a ferroelectric imagingsystem which is both easy and economical to fabricate.

Another object of the invention is to alleviate the shunt capacitancealong any column of detector elements and to reduce substantially anyfixed pattern noise.

Still another object of the invention is to eliminate substantially thepossibility of thermal shorts existing between the detector matrix andits substrate or support member.

Yet another object of the invention is to eliminate the necessity formulti-ball bonds to bond the sense lines to the preamps.

Still yet another object of the invention is to increase the speed withwhich any one line or detector can be addressed.

Briefly stated, the invention comprises an improved ferroelectricimaging system which includes a chopper, a lens system and a housingwith a window, a temperature controlled heat sink, a silicon switchingmatrix, an anisotropic ferroelectric heat sink, and an improved detectormatrix means. The window is mounted in one end of the housing in opticalalignment with the chopper and lens system. The temperature controlledheat sink is mounted in the end of the housing opposite the windowbearing end. The temperature controlled heat sink supports within thehousing the silicon switching matrix on top of which is formed theanisotropic heat sink which in turn supports the detector matrix in aspaced relationship to the window. The space between the ferroelectricwafer and window comprises a vacuum or a dry inert atmosphere.

The novel features believed to be characteristic of this invention areset forth in the appended claims. The invention itself, however, as wellas other objects and advantages thereof may best be understood byreference to the following detailed description of illustrativeembodiments when read in conjunction with the accompanying drawings inwhich:

FIG. 1 is a block diagram of the improved ferroelectric imaging system;

FIG. 2 is a cross-sectional view of the detector mechanism for theferroelectric imaging system;

FIG. 3 is a view, partly schematic, showing one embodiment of theferroelectric imaging system;

FIG. 4 is a cross-sectional view of a field effect transistor formingthe switching matrix of the ferroelectric imaging system;

FIGS. 5a-5c are cross-sectional views of the anisotropic heat sinkconfigurations of the ferroelectric imaging system;

FIGS. 6a-6d show the fabrication of the anisotropic heat sink of FIG.5a;

FIGS. 6e-6g show the fabrication of the anisotropic heat sink of FIG.5b;

FIGS. 6h-6i show the fabrication of the anisotropic heat sink of FIG.5c;

FIGS. 7a-7c depict fabrication of one embodiment of the ferroelectricdetector matrix;

FIGS. 8a-8c depict fabrication of a second embodiment of theferroelectric detector matrix;

FIG. 9 is a view, partly in cross section, of one embodiment of thedetector mechanism with the housing removed;

FIG. 10 is the electrical equivalent circuit of the detector matrixembodiment of FIG. 9;

FIG. 11 is a view, partly in cross section, of a second embodiment ofthe detector mechanism with the housing removed;

FIGS. 12a-12c are views, partly schematic of configurations of thesecond embodiment of the ferroelectric imaging system; and

FIG. 13 is the electrical equivalent circuit of the detector mechanismembodiment of FIG. 11.

Referring now to FIG. 1, the thermal imager system comprises an infraredenergy chopper 10, lens system 12, detector assemby 14, drive and readout electronics 16, video processor 18, timing and control means 20, anddisplay means 22. The chopper 10 is, for example, a mechanical chopperfor interrupting the flow of infrared energy to the surface of detectormatrix of the detector assembly 14. The lens system 12 may be, forexample, an infrared lens system having an object lens, correction lens,and focusing lens for focusing thermal energy emanating from a scene(not shown), which is chopped by the chopper 10, onto the detectormatrix of the detector assembly 14. It will be understood by thoseskilled in the art that various types choppers and lens systems existcapable of performing the desired optical functions. The detector matrixof the detector assembly 14 includes a plurality of thermal energysensors for producing electrical signals representative of the thermalenergy impinging on the sensors for the video processor 18. The detectormatrix of the detector assembly 14 is more fully described hereinafter.The video processor 18 processes the signals representative of the sceneinto display signals for display by the display means 22. The displaymeans 22 may be, for example, a cathode ray tube, and the videoprocessor 18 is that used for processing electrical signals into a TVformat. The drive and read out electronics 16 is connected to thedetector matrix of the detector assembly 14 for biasing the thermalsensors and reading out electrical signals representative of the scene.The timing and control mechanism 20 guides the action of the chopper 10,drive and read out electronics 16 and video processor 18 to selectivelyproduce signals of the scene in a desired format for display by displaymeans 22.

Referring now to FIG. 2 for a description of the detector assembly 14used in the ferroelectric imaging system, the detector assembly 14comprises housing 26 which may be, for example, a stainless steelhousing having a window 28 formed in one major surface. The window, forthermal energy, may be of, for example, a variety of glasses, forexample, silicon, chalcogonide, etc., which transmit from the near IRthrough gamma rays, and germanium or indium which transmit,respectively, in the 8-14 micron and 35 micron infrared wavelengths. Thewindow 28 is positioned in the light or thermal path to the detectormatrix 24. A heat sink 30, which may be, for example, a ceramicsubstrate of beryllium oxide, closes the other end of housing 26.Heating elements 32 are connected to a source of power (not shown) forcontrolling the temperature of the heat sink 30. A switching matrix 34,which may be, for example of silicon, is mounted on the temperaturecontrol heat sink 30. An anisotropic heat sink 36 is formed on thesilicon switching matrix 34, and the detector array or matrix 24 ismounted on the anisotropic heat sink 36. A space 40 which may be, eithera vacuum or a dry inert atmosphere is provided intermediate the detectormatrix 24 and window 28. The silicon switch matrix 34, anisotropic heatsink 36 and the detector matrix 24 will be described hereinafter indetail.

The silicon switching matrix 34 (FIG. 2), in one embodiment, comprises aplurality of solid state switches 42 (FIG. 3), which may be, forexample, field effect transistors (FET). A FET switch 42 (FIG. 4) isprovided for each element 44 of the detector matrix 24. The siliconswitching matrix of FET switches 42 are made by diffusing for eachswitch two N-regions called the drain 46 and the source 48, side by sideinto the surface of a P-type silicon slice 50. A layer of silicon oxide52 is grown over the surface. A pad 54 and strip 56 of metallization aremade to penetrate through windows in the oxide 52, to contact thesilicon 50. A third metal strip 58, the gate, lies on top of the oxide52, over the gap between the N-regions. In another embodiment the gate58 is a dual gate 58 and 58' the ends of which are the dashed lines ofFIG. 4. FET's so constructed are of the enhancement type, referring tothe increase of working-current conduction by the control voltageapplied to the gate. That is the switch is off until a voltage isapplied to the gate. The drain pads 54 form the base for the conductingrods of the anisotropic heat sink. As the methods of fabricating FETswitches in matrix format are well known to those skilled in the art,the techniques are not described in further detail.

The anisotropic heat sink 36 (FIG. 2), shown in FIGS. 5a-5c, comprises avacuum surrounding a plurality of spaced conductor rods 60 formed on thedrain pads 54 of the FET switches 42. The electrical resistance of theconductor rods or leads 60 must be low; while on the other hand thethermal resistance must be high. This is necessary as the thermalconductivity of silicon in which the FET switches 42 are formed is highand acts as a thermal short between the detector elements 44 if athermal impedance is not provided. Thus, the thermal resistance of theconducting rods 60 is the dominant factor in terms of importance. Thethermal conductivity is defined by the expression for perpendicular heatflow through a surface; that is,

    H = kA (∂T/∂n)

where:

H = heat

k = conductivity

A = cross sectional area of the conducting rod

∂T/∂n = temperature gradient at the surface.

For purposes of discussion, the expression can be simplified to:

    H = -KΔT

where:

K = thermal conductance of the element and is given by:

    K = kA/L

where:

L = length of the conducting rod over which the temperature differenceis measured and ΔT is the temperature difference.

From the last formula it is evident that two ways exist to achieveeffective low thermal conductance. First, the K is made small by choiceof materials; secondly, the aspect ratio, A/L, is made small byfabrication techniques. As the range of values of k is rather restrictedbecause of the electrical conductivity and fabrication techniques, thesecond way is preferred. The same conclusion is supported by the totalconductivity formula:

    K.sub.e = K.sub.c + K.sub.i = k.sub.c (A.sub.c /L) + k.sub.i (A.sub.e -A.sub.c)/L

where the subscripts e, c and i refer to the total detecting elementregions, conducting region, and insulating region, respectively. Ask_(i) can be made much less than k_(c), the requirement for low K_(e)can best be met by having the aspect ratio, A_(c) /L, very small. Thus,by keeping the cross sectional area of the conducting rods small ascompared to the length, the aspect ratio of the conducting rod of eachdetector element will be small. For example, where the anisotropic heatsink is a vacuum and the detector center to center spacing is four mils,a conducting rod that is one mil in length and 0.16 square mils in crosssectional area has a factor of 100 reduction in thermal conductance,K_(e).

Referring now to FIG. 6a, the drain pads 54, gates 58, and source pads56 are coated with a suitable conductor rod defining material 62 suchas, for example, a photoresist sold under the trademark RISTON M811 byE. I. duPont de Nemours & Company. Next, a mask 64 (FIG. 6b) is placedover the layer of photoresist 62 and the photoresist exposed toultraviolet light. After exposure, the image of the mask is developedand the photoresist areas not exposed to light are removed, leaving thesource pads 54 bare in those areas. Next the conducting rods 60 (FIG.6c), which may be, for example, nichrome, nickel, or gold, are formedeither by vacuum deposition or electroplating. Vacuum deposition can beused if the height for given operation is small, for example, 0.05 mils.However, even this height requires many operations and the possibilityof opens is high. Hence, electroplating is preferred, even though theselection of materials for the conducting rods is reduced and theapplication of a negative potential to the drain pads 54 is difficult.

For electroplating the negative potential relative to the potential ofthe bath is, for example, applied to the pads 54 by turning on theresistors. Another approach is to use an alternate metal mask in lieu ofthe drain defining pads 54. In this technique the entire surface ismetalized prior to the application of the photoresist to form anelectrode with a contact at the edge of the chip. The photoresistpattern is then laid down and the rods built up by electroplating. Therod tips or pads 66 (FIG. 6d) are then polished, if necessary, to makethem smoother and of more uniform heighth and the photoresist removed.If the metal surface electrode technique is used the metal between theconductor rods 60 is etched away. This method of fabrication isadvantageous where the rod thickness provides the mechanical strengthnecessary to support the detector structure.

Where the length of the conductor rods 60 is such that the photoresistpattern cannot be made with the required rpd diameter, multipledepositions are used (FIG. 6e). For each additional rod segment 68 therod pads 66 (FIG. 6d) are removed and an additional layer 70 of aphotoresist (FIG. 6e) is applied. The layer 70 is masked with a mask 72.To facilitate alignment of the rod patterns, the mask rod diameters aremade slightly larger than the ends of the conducting rods 60. Thus, whenaligned the rod defining rings will be seen one within the other. Thephotoresist 70 is then exposed by ultraviolet light and the exposedphotoresist removed to form the pattern for rod segments 68. The rodsegment 68 (FIG. 6f) are then formed by electroplating as previouslydescribed. The tips or pads 74 (FIG. 6g) are then polished, ifnecessary, to make them smooth and of uniform height. After polishingthe photoresist is removed.

Where the mechanical strength of the conducting rods 60 is insufficientto support the detector matrix 24, a glass filled photoresist 76 (FIG.6h) is used as a back fill about the conductor rods 60. The photoresistis then exposed by ultraviolet light and the photoresist removed toleave the glass particles 78 in place. The glass particles 78 are thensintered by backing in an oven to imbed the conducting rods 60 (FIG. 6i)and provide the mechanical strength necessary to support the detectorstructure.

The detector matrix 24 (FIG. 7b) comprises a wafer of ferroelectricmaterial 80 having longitudinal lead patterns 82 (FIG. 7a) and pads 84(FIG. 7c) formed on opposing major surfaces by evaporation/etchingtechniques. The leads 82 are column leads formed on the top surface ofthe ferroelectric wafer 80 and the pads 84 are leads formed on the lowersurface of the ferroelectric wafer 80. The pads 84 and the crossoverportions of the lead patterns 82 together with the ferroelectricmaterial 80 form the elements of the detector matrix 24. The resultingdetector matrix 24 is a plurality of capacitor type thermal sensors 44represented by the squares of FIG. 7b.

The ferroelectric wafer 80, i.e., a material having dielectricproperties, preferably is one having a Curie temperature near ambienttemperature (0° to 150° C.) such as, for example, barium titanate (BaTi₂O₃), triglycenesulfate (TGS), barium strontium titanate, lead titanate,potassium dihydrogen phosphate, and Rochelle salt. Barium titanate,which has a Curie temperature of about 125° C., is preferred.

Whichever ferroelectric material 80 is selected it must be used near itsCurie temperature, i.e., the temperature above which a ferroelectricmaterial ceases to be spontaneously polarized. Above the Curietemperature, barium titanate is a cubic crystal structure stronglysusceptible to polarization. Thus, its dielectric constant is a strongfunction of the temperature. On the other hand at temperatures wellbelow the Curie temperature ferroelectric material exhibits aspontaneous dipole moment. This produces an internal electric field andsurface charges then accumulate to quench the field. As the degree ofpolarization is temperature dependent, operation well below the Curietemperature is the basis for the pyroelectric vidicon which is not apart of this invention.

Referring now to FIGS. 8a-8c, another embodiment of the detector matrix24 is disclosed. In this embodiment the electromagnetic wafer 80, whichmay be that of FIG. 7b, has its upper surface plated with a metalconductor 86 (FIG. 8a). The metal conductor 86 is in electrical contactwith bus bias leads 88 and 90. Biasing leads 88 and 90 are disposedadjacent opposite edges of the plate 86 for connection to a source ofbias current for charging the upper plate 86. Metal pads 92 (FIG. 8c)are deposited on the lower surface of the ferroelectric wafer 80. Metalpads 92, together with the ferroelectric wafer 80 and the upper plate 86form the elements 44 of the detector matrix (FIG. 8b).

The combination of the detector matrix 24, anisotropic heat sink 36 andswitching matrix 34 for the first embodiment is shown in FIG. 9. Theconducting rods 60 of the anisotropic heat sink 36 are grown on thedrain pads 54 of the switching matrix switches 42. The tips or pads 66of the conducting rods 60 are attached, for example, by bump bonding tothe lower pads 84 of the detector matrix 24. The stripes 82, which alsoform the sense lines, are connected to the inputs of the amplifiers byball bonding, for example.

Referring now to FIG. 3, the FET switches 42 of each row have theirgates 58 connected by leads 55 to a row (y) address circuit 94 and theirsources 48 connected by leads 96 to a biasing (V_(Bias)) source ofpower. The FET switches 42 have their drains 46 connected by theanisotropic heat sink connecting rods 60 to the lower lead stripes orplates 84 of the ferroelectric capacitors 44 forming the detector matrix24. Each lower lead 84 interconnects a column of capacitors. The upperlead stripes or plates 82 of each column of capacitors 44 are ballbonded to leads 98 which are connected to preamplifiers. Thepreamplifiers 100 are multiplexed by an x addressing circuit 95 to readthe amplified electrical signals of each capacitor 44 to the videoprocessor 18. The output of the video processor 18 is connected by lead102 to the display 22.

In operation, the y addressing circuitry 94 provides a high voltage tothe gates 58 of row one switches 42 and a low to the gates of the otherrows of switches. The high voltage, applied to gates 58, turns on theswitches 42 of row one to charge the ferroelectric capacitors 44 to avoltage (E) (FIG. 10) through a resistor R. At the turn on time thechopper 10 (FIG. 1) is sequentially interrupting the infrared energyemanating from the scene and impinging on the capacitors 44. The FETswitches 42 (FIG. 3) of row one are turned off sequentially at the timethe chopper is admitting the infrared energy sequentially (from left toright) to the ferroelectric capacitors 44 and their capacitance ischanged in proportion to the infrared energy impinging thereon (V_(c)).Then just before the chopper sequentially interrupts the infrared energyagain the switches 42 of row one are closed and the x addressing circuit95, which is synchronized with the chopper, sequentially addresses eachcapacitor 44 of row one. Each capacitor discharges down to the voltage E(FIG. 10) which is supported by the biasing voltage applied by theclosing of the switches 42. The output, therefore, is equal to theintensity of the infrared energy impinging on the capacitors. (V = EV_(c)). Since V_(c) is the voltage across C just before the switch wasclosed

V_(c) = Q(a)/c(Υ) where Q(o) is the original charge placed on C, andC(Υ) is the C value at time (Υ). Further, Q(o) = EC(o), thus V = E - EC(o)/C(Υ) or V = E[C(Υ) - C(o)/C(Υ)].

The outputs on lines 98 are individually buffered by the preamplifiers100, multiplexed by the x addressing circuit 94 and processed in thevideo processor 18 to provide single line video corresponding to thesignal level for each capacitor 44 in the row for display by display 22.The process is repeated for each row of the detector matrix 24. It willbe appreciated that the resulting single line video information is ofpositive polarity on the first frame and of negative polarity on thesecond frame. An external polarity reversing circuit (not shown) is usedto invert the polarity of all the negative polarity frames.

Referring now to FIG. 11, for a description of a second embodiment ofthe detector assembly 14 of the ferroelectric imaging system, thedetector matrix 24 is that shown and described above in connection withFIGS. 8a-8c. The detector matrix 24 is attached, for example, by ballbonding to the tips or pads 66 or 74 of the conducting rods 60 of theanisotropic heat sink 36. The conducting rods 60 are built up from thedrain pads 54 and the FET switches of the switching matrix 34 have theirdrains connected one on one to the detection elements of the detectormatrix 24. The gates of the FET switches 42 are dual gates 58 and 58';gates 58 are connected by columns to the x address circuit 95 and gates58' are connected by rows to the y address circuit 94. The source 48 ofthe FET switches 42 are connected by rows by leads 106 to bufferamplifier 108 and to drains 110 of precharge FET switches 112. The gates114 of the precharge FET switches 112 are connected to precharge voltagesources, and the sources 116 of these switches are connected to areference voltage. The housing 26 and temperature controlled heat sink30 (FIG. 2) are also used for the embodiment shown in FIG. 11 and arenot shown.

Referring now to FIG. 12a-12c for three configurations of a secondembodiment of the ferroelectric imaging system. In each case the x lines98 and y lines 55 are addressed synchronously with the x lines 98 beingthe low speed lines. The infrared energy emanating from a scene ischopped by chopper 10 (FIG. 1). The chopper edge 10 is moving in the xdirection. The action of the chopper causes each detection element ofthe detector matrix 24 to be either heated or cooled according to thedifference in the scene radiance for that particular detection element.The heating or cooling cycle causes a change in that capacitor elementwhich is to be sensed and converted to video by the switching matrix 34.The change in capacitance will be at its maximum for each element as thechopper edge passes that element. Hence, the sampling process of theswitching matrix is synchronized to the passage of the chopper.

Referring to the configuration of FIG. 12a, each ferroelectric capacitor44 is x and y addressed by two gates 58 and 58' of its corresponding FETswitch 42 beginning with the switch 42A and capacitor 44A in the upperleft corner. At system start up the x₁ line 98 is addressed as thechopper moves by to turn on gates 58 of the switches 42 in column x₁.Once these gates 58 are turned on then the y₁ line 55 is addressed andgates 58' of row y₁ are turned on. At this time there is a lowresistance path from the drain of switch 42A through line 106 to thebuffer amplifier 108. When the low resistance path is established to thebuffer amplifier, the high speed precharge FET switch 112 is pulsed toestablish reference voltage V_(ref) on the sense line 106. Thisswitching action establishes a voltage of (V_(ref) - V_(bias)) acrossthe ferroelectric capacitor. Then, the high speed precharge switch 112is turned off and the y₂ line 55 addressed and the precharge switchaction repeated to establish the voltage (V_(ref) - V_(bias)) across thecapacitor 42B. This x-y addressing sequence is repeated until all of they lines 55 have been activated and the leading edge of the chopper hasmoved to the x₂ line 98. At this time the x₂ line 98 is charged and they lines 55 multiplexed for precharging the detector elements of the x₂line. The process is continued until all the detector elements have beenpreset.

Once all of the detector elements have been preset in sequence and thenext leading edge of the chopper has reached the position correspondingto the x₁ line 98, the read-out cycle begins. The x₁ line 98 ismultiplexed on and the high speed precharge switch 112 is pulsed. Thischarges all the parasitic capacitances on the sense line 106 to V_(ref).Then the high speed precharge switch is turned off to isolate the senseline 106. The y₁ line 55 is then addressed to establish the lowresistance path again from the drain of switch 42A through the senseline 106 to the buffer amplifier 108. It will be appreciated that therehas been one field time elapsed between the setting and sampling of theferroelectric capacitor 44A. During this time the trailing edge of thefirst chopper element has passed the x lines 98 and the element 44A hasbeen flooded with the infrared energy from the scene and its capacitancehas changed accordingly. The change in detector element capacitanceforces a change in the voltage input to the buffer amplifier 108proportional to the change in the capacitance because of the impingingradiation. it will be appreciated that the synchronization of thechopper position, i.e., from closed to open, can be reversed.

The timing and control circuit 20 (FIG. 1) generates the requiredsignals for detector addresssing, display drive and choppersynchronization. There are a variety of ways in which this isaccomplished. One approach is to use as the basic elements (1) anoscillator, (2) a high speed count down circuit clocked from theoscillator for pixel addressing and line rate signals, (3) a low speedcount down for line addressing and field rate signal, and (4) a phaselock loop for synchronizing a mechanical chopper to the field ratesignals. The display horizontal and vertical sweep signals are generatedfrom the line and field signal, respectively.

The magnitude of the change in voltage is derived from the electricalequivalent circuit of the system (FIG. 13). In this circuit the dualgated FET switch 42 is shown as a single pole switch Sp, the high speedprecharge FET switch 112 as switch S_(PC), the detector elementcapacitance as capacitor C_(D), the precharge FET switch 112 parasiticcapacitance and the amplifier input capacitance as C_(PG), the detectorelement parasitic capacitance introduced by the bonding operation asC_(PD), and the sense line 106 parasitic capacitance as C_(PS). When thecapacitors 44 of the ferroelectric detector 14 are precharged allswitches are closed (C). The charge (Q(c)) on each of the capacitors is:

    Q.sub.D (c) = C.sub.D (c)(V.sub.ref - V.sub.bias)

    Q.sub.PD (c) = C.sub.PD V.sub.ref

    Q.sub.PG (c) = C.sub.PS V.sub.ref

The total charge Q_(T) (c) is:

    Q.sub.T (c) = Q.sub.D (c) + Q.sub.PD (c) +Q.sub.PS (c)

where

C_(d) (c) is the capacitance of the detector element when the chopper isclosed.

The switches are then opened (o) and during the next field the highspeed FET switch (S_(PC)) is closed and opened so that the parasiticcapacitance C_(PG) is reset to C_(PS) V_(ref). The charge Q_(PD) andQ_(D) values have been held constant while the capacitance (C_(D)) onthe detector element has changed from C_(D) (c) to C_(D) (o).

Next the FET switch S_(P) is closed to allow the capacitances C_(D),C_(PD) and C_(PG) to reach equilibrium to provide new voltage to thebuffer amplifier 108. Mathematically this is:

    Q.sub.D (o) = C.sub.D (o) (V.sub.G - V.sub.bias)

    Q.sub.PD (o) = C.sub.PD V.sub.G

    q.sub.ps (o) = C.sub.PS V.sub.G

with Q_(T) (o) = Q_(D) (o) + Q_(PD) (o) + Q_(PS) (o)

Now Q_(T) (o) = Q_(T) (c)

such that

    C.sub.D (c) [V.sub.ref - V.sub.bias ] + C.sub.PD V.sub.ref + C.sub.PS V.sub.ref = C.sub.D (o) [V.sub.G - V.sub.bias ] + C.sub.PD V.sub.G + C.sub.PS V.sub.G

    vg=[c.sub.p (o) - C.sub.C (c) ] V.sub.bias + [C.sub. (c) + C.sub.PD + C.sub.PS ] V.sub.ref/ [C.sub.D (o) + C.sub.PD + C.sub.PS ]

the signal voltage is the change in the voltage at the amplifier inputwhich is:

    V.sub.sig = V.sub.G - V.sub.ref = [C.sub.D (o) -C.sub.d (c)]V.sub.bias + [C.sub.D (c) + C.sub.DP + C.sub.PS ] V.sub.ref - [C.sub.D (o) - C.sub.DP + C.sub.PS ] V.sub.ref/ C.sub.D (o) + C.sub.PD + C.sub.PS ] = [C.sub.D (c) - C.sub.D (o) ] [V.sub.ref - V.sub.bias ]/[C.sub.D (o) + C.sub.PD + C.sub.PS ]

the voltage signal (V_(sig)) is proportional to the change incapacitance resulting from the chopper opening and closing and is sensedby the video processor 18 and fed to the display 22 for that particularpixel element.

Before the switch (S_(p)) is opened, S_(pc) is closed and opened. Thisaction sets the detection element charge to:

    Q.sub.D (o) = C(o) [V.sub.ref - V.sub.bias ].

Hence, in the next field when this element is addressed, the voltagesignal (V_(sig)) will have the same magnitude for the same scene but ofopposite sign. For this reason the video processor 18 contains afield-to-field phase reversal circuit. It will be recognized thatresetting a capacitor, for example, 44A is equivalent to precharging thesense line 106 prior to reading the succeeding capacitor 44B. Thus,after the precharge switch 112 (S_(pc)) is opened switch 42A is openedand switch 42B is closed, allowing the signal to be displayed forcapacitor 44B. This action sequence continues for the entire field.

An example of a field-to-field reversal circuit is as follows. A frameof information is comprised of two fields -- one for chopper closed andone for chopper open. As stated the signal information has oppositepolarity during the two fields. A set of signals corresponding to thefield rate referenced as FLD and FLD is generated by the timing andcontrol circuit. FLD will have a "high" level during chopper open and"low" level during chopper closed. The FLD signal has the oppositelogical sense. The signal line video are fed to both the positive andnegative inputs of an instrumentation amplifier and the signals areswitched. The resulting video have consistent signs.

Referring now to the configuration of FIG. 12b, the double gates 58-58'of the switches 42 for addressing the ferroelectric capacitors 44 of thedetector matrix 24 has been replaced by a single gate 58. Thus, when therow x₁ is addressed by the addressing circuit 95 each switch thereof isclosed and the corresponding capacitor 44 is simultaneously connected toits sense line 106, precharge switch 112, and preamplifier 108. Theoutputs of each preamplifier 108 is connected to the input of the bufferamplifier 130 through their multiplexer switches 120. The gates 122 ofthe y₁ -y_(n) multiplexer switches are connected to outputs y₁ -y_(n) ofthe y addressing circuitry 94. The drains 124 of the multiplexerswitches 120 are also connected by lead 126 to the drain of prechargeFET switch 128 and buffer amplifier 130. Buffer amplifier 130 isconnected by lead 132 to the video processor 18, which is connected tothe display 22.

In operation, the precharge action is done at the x line scan rate, andthe detector capacitors of each line now provide their information atthe same time at the output of its amplifier 108. The lines ofinformation are then multiplexed out through the buffer amplifier 130 bythe y multiplexing switches 124. As in the configuration of FIG. 12a they lines are cycled ON after each x line.

Referring now to FIG. 12c for a third configuration of the secondembodiment of the ferroelectric imaging system. This configuration is acombination of the configurations of FIGS. 12a and 12b. In the thirdconfiguration the preamplifiers 108 and capacitors 118 of FIG. 12b havebeen omitted and the buffer amplifier 130, which is connected by lead126 to the y address switches 124, acts also as the preamplifier.

Although several embodiments of this invention have been describedherein, it will be apparent to a person skilled in the art that variousother combinations and modifications to the details of constructionshown and described may be made without departing from the scope of thisinvention.

What is claimed is:
 1. An improved ferroelectric imaging systemcomprising:(a) a chopper for alternately interrupting infrared energyemanating from a scene; (b) a lens system for focusing the infraredenergy emanating from a scene; (c) a ferroelectric detector matrix inthe optical path of the lens system for receiving the alternatinginfrared energy, said ferroelectric detector matrix including aferroelectric wafer, a layer of electrical conducting material coveringsubstantially a first side of the ferroelectric wafer, busbars formed onthe layer of electrical conducting material for connection to a biasingsource of power for charging the layer of electrical conductingmaterial, a plurality of electrical conducting pads arranged in rows andcolumns on a second side of the ferroelectric wafer, the plurality ofelectrical conducting pads forming a plurality of lower plates, saidlayer of electrical material, ferroelectric wafer, and plurality ofelectrical conducting pads forming a plurality of ferroelectriccapacitors arranged in rows and columns; (d) a temperature controlledheat sink for maintaining the ferroelectric wafer substantially at itsCurie temperature for operation; (e) a switching matrix comprising aplurality of solid state switches including a switch for eachferroelectric capacitor, a drive and read out electronics including atiming and control circuit for selectively opening and closing theplurality of solid state switches synchronously with the chopper action,a plurality of switch lines, said plurality of solid state switchesconnecting rows of the plurality of ferroelectric capacitors to theplurality of sense lines, said sense lines connected to a referencevoltage and operative in response to selected switching action to readout signals representative of the infrared energy impinging on theferroelectric capacitors, and at least one preamplifier for amplifyingthe electric signals; (f) a video processor operative in response to theamplified electrical signals for providing electrical signals in adisplay format; and (g) a display connected to the video processor fordisplaying the electrical signals representative of the infrared imageof the scene.
 2. An improved ferroelectric imaging system according toclaim 1 wherein the switching matrix comprises a plurality of fieldeffect transistor (FET) switches, each FET switch having a source, adrain, and first and second gates, said plurality of FET switches havingtheir drains connected to the lower plates of the plurality offerroelectric capacitors, their sources connected to the plurality ofsense lines and their first gates connected in columns to a x addressingcircuit of the drive and read out electronics, and their second gatesconnected in rows to a y addressing circuit of the drive and read outelectronics.
 3. An improved ferroelectric imaging system according toclaim 1 wherein the switching matrix comprises a first plurality offield effect transistor (FET) switches arranged in rows and columns,said rows of FET switches having their drains connected to the lowerplates of the ferroelectric capacitors and their sources connected to asense line, and said columns of FET switches having their gatesconnected to a x addressing circuit of the drive and read outelectronics, a second plurality of FET switches having their drainsconnected to a reference voltage, their gates connected to a low speedcircuit of the timing and control circuit and their sources connected tothe sense lines of the rows of FET switches, said second plurality ofFET switches operative in response to timing signals for sequentiallyapplying a voltage reference signal to the sense lines where it combineswith the output of the ferroelectric capacitors for providing electricalsignals representative of the infrared energy, and a third plurality ofFET switches having their sources connected to the sense lines, theirgates connected to a y address circuit of the drive and read outelectronics, and their drains connected to a preamplifier, said thirdplurality of FET switches operative in response to the y addresses forselectively coupling the electrical signals representative of theinfrared energy impinging on the ferroelectric capacitors to thepreamplifier.
 4. An improved ferroelectric imaging system according toclaim 3 further including a plurality of amplifiers operativelyconnected to the outputs of the plurality of sense lines for amplifyingthe electrical signals representative of the infrared energy impingingon the ferroelectric capacitors prior to read out by the third pluralityof FET switches.
 5. A method of ferroelectric imaging comprising:(a)biasing a plurality of ferroelectric capacitors of a ferroelectricdetector matrix with a biasing voltage; (b) closing a chopper tointerrupt infrared energy emanating from a source; (c) closingsequentially x address gates of rows of FET switches at a first rate ofspeed; (d) closing sequentially y address gates of columns of FETswitches at a second rate of speed, said second rate of speed beingfaster than the first to establish sequentially a low resistance pathfrom the ferroelectric capacitors to a reference voltage switch; (e)closing a reference voltage switch substantially simultaneously with theclosing of the y address gates to charge sequentially the ferroelectriccapacitors of the columns up to the reference voltage; (f) opening thechopper to pass infrared energy emanating from the scene; (g) openingsequentially the x address gates and the reference voltage whereby thecapacitance of the ferroelectric capacitors is varied in proportion tothe impinging infrared energy; (h) closing and opening the x addressgates simultaneously with the closing and opening of the voltagereference switch and interruption of the infrared energy impinging onthe ferroelectric capacitors by the chopper to read out electricalsignals proportional to the impinging infrared energy; (i) amplifyingthe electrical signals to a working level for a video processor; (j)processing the electrical signals in a video processor; and (k)displaying the video signals to form a visible infrared image of thescene.
 6. A method of ferroelectric imaging comprising:(a) biasing aplurality of ferroelectric capacitors of a ferroelectric detector matrixwith a biasing voltage; (b) closing and opening a chopper to interruptinfrared energy emanating from a scene and impinging on theferroelectric capacitors; (c) closing, during the closing of thechopper, a plurality of first reference voltage switches and closingsequentially x address gates to close sequentially columns of thecapacitor switches to charge sequentially columns of ferroelectriccapacitors to a reference voltage; (d) opening sequentially the xaddress gates to open sequentially columns of the capacitor switchesduring the opening of the chopper whereby the infrared energy impingingon ferroelectric capacitors acts to vary the capacitance in proportionto the intensity thereof for producing signal voltages representative ofthe impinging infrared energy; (e) closing and opening sequentially thex address gates to close and open sequentially columns of the capacitorswitches to provide a low impedance path from the ferroelectriccapacitors to preamplifiers for the signal voltages; (f) amplifying thesignal voltages sequentially by columns; (g) closing and openingsequentially the y address gates synchronously with a second voltagereference switch during x addressing of the columns of ferroelectriccapacitors to read out the amplifier signal voltages; (h) videoprocessing the signal voltages into video signals; and (i) displayingthe video signals to produce a visible representation of the scene.
 7. Amethod of ferroelectric imaging comprising:(a) biasing a plurality offerroelectric capacitors of a ferroelectric detector matrix with abiasing voltage; (b) closing and opening a chopper to interrupt infraredenergy emanating from a scene and impinging on the ferroelectriccapacitors; (c) closing, during the closing of the chopper, a pluralityof first reference voltage switches and closing sequentially x addressgates to close sequentially columns of capacitor switches to chargesequentially columns of ferroelectric capacitors of the ferroelectricdetector matrix to a reference voltage; (d) opening sequentially the xaddress gates to open sequentially columns of the capacitor switchesduring the opening of the chopper whereby the infrared energy impingingon the ferroelectric capacitors acts to vary the capacitance inproportion to the intensity thereof for producing signal voltagesrepresentative of the impinging infrared energy; (e) closing and openingsequentially the x address gates to close and open sequentially columnsof the capacitor switches to provide a low impendance path from theferroelectric capacitors for the signal voltage representative of theimpinging infrared energy; (f) closing and opening sequentially the yaddress gates synchronously with a second voltage reference switchduring the x addressing of the columns of ferroelectric capacitors toread out the signal voltages; (g) video processing the signal voltagesinto video signals; and (h) displaying the video signals to produce avisible representation of the scene.